OS2B-4:DETERMINING THE OCCURRENCES OF GAS HYDRATE IN THE NORTHERN GULF OF MEXICO USING PETROLEUM INDUSTRY WELL LOGS

Majumdar URMI 1, Cook ANN 1, Frye MATTHEW 2  and  Shedd WILLIAM 2
1,The Ohio State University School of Earth Sciences, USA; 2 Bureau of Ocean Energy Management, USA

    We take a new approach to evaluating the presence of natural gas hydrate in the northern Gulf of Mexico, offshore southern United States, by analyzing well logs from 1500 petroleum industry wells that penetrate the gas hydrate stability zone (GHSZ).  These wells extend over 300,000 km2 and mainly include wells drilled from 1994 to 2011 by various operators.   When this research is completed, we will provide a new estimate of the amount of natural gas hydrate in the northern Gulf of Mexico and estimate the likelihood that any given well will penetrate a gas hydrate reservoir.

    In the GHSZ, which is the shallowest section of the industry well, the types of logs typically available are gamma ray and resistivity.  Gamma ray logs measure the natural radiation of the sediment and qualitatively indicate either sand or clay sediment.  Induction resistivity and/or propagation resistivity measure the resistivity of the bulk sediment system.  In shallow water-saturated sediments in the GHSZ, resistivity is typically close to 1 ohm*m, though this number can vary. Increases in resistivity beyond the background resistivity may indicate natural gas hydrate. In addition, separation between different propagation resistivity logs can indicate the presence of natural gas hydrate in near-vertical fractures.

    Preliminary results are available from three Gulf of Mexico oil and gas protraction areas: Alaminos Canyon (AC), East Breaks (EB) and Keathley Canyon (KC). Presence of gas hydrate was indicated in all the three areas, though the number of total wells in Keathley Canyon area is significantly lower than that in  Alaminos Canyon and  East Breaks. Most of the hydrates found in these sites occur in near-vertical fractures in clay sediments in what is most likely low to moderate saturations of gas hydrate. Gas hydrate also appears in some sand reservoirs. Based on our assessment, at least 17 wells in East Breaks, 20 wells in Alaminos Canyon and 3 wells in Keathley Canyon may contain gas hydrates.

In  Keathley Canyon only three wells in two blocks are noted as having possible natural gas hydrate occurences.   Block KC 151 was drilled by the Gulf of Mexico Gas Hydrate Joint Industry Project (JIP) and contains natural gas hydrate in near-vertical fractures through a total of 75 meters of subsurface clay and sandy clay intervals.  A well in KC 291 may also have a number of gas hydrate bearing intervals in both sand and clay.

    The most interesting well in East Breaks is in block EB 990. Presence of gas hydrate is likely in a total of 173 meters in clay within the GHSZ. The 40 inch phase-shift resistivity log measures 2 ohm*m to 10 ohm*m in these gas hydrate intervals as opposed to the clay background resistivity measure of about 1 ohm*m.  The propagation resistivity curve separation in EB 990 is characteristic of near-vertical gas hydrate filled fractures, where the deepest penetrating propagation resistivity logs measure the highest resistivity. The other potential gas hydrate bearing wells appear in blocks EB 712, EB 946, EB 994, EB 602, EB 832, EB 558 and EB 642.

    AC 857 is the most interesting block in Alaminos Canyon with possible gas hydrate shows in eleven wells; however, it is possible that the elevated resistivity in these wells is due to the presence of gas condensate.  The most significant well may contain 229 meters of possible gas hydrate show.  On an average 91 meters of gas hydrate appears in each well. Majority of these wells contain one or more intervals of high-angle gas hydrate filled fractures in clay. These high-angle gas hydrate filled clay intervals show a very high resistivity of 20 ohm*m on an average on deep-phase propagation resistivity or 40 inch phase-shift resistivity logs against the average background clay resistivity of 1 ohm*m.  Propagation resistivity curve separation is also observed. The other interesting wells in Alaminos Canyon include AC 810, AC 557, AC 818, AC 856, AC 815 and AC 21.